1. Field of the Invention
The present invention relates to a communication system, and in particular to a communication system having bus-connected nodes.
2. Description of the Related Art
Some existing related art systems employ a CAN (Controller Area Network) standard as a serial data bus standard. One of the features of the CAN standard is an arbitration method which is invoked when a collision between message packets occurs. A typical arbitration method as used in the CAN standard will be described below.
Referring to FIG. 1, an exemplary frame structure of a message packet for use in the CAN standard is shown, in which an arbitration field 102 indicates an ID that is uniquely defined for each device. Such an ID as indicated in the arbitration field represents an ID of a destination device to which the message packet is to be sent. Each device determines, based on a comparison between its own ID and the ID indicated in the arbitration field of a message packet flowing along a bus, whether or not the message packet is to be received.
Such IDs indicate priorities of respective devices in a bus system, and are used in an arbitration when a collision between message packets occurs on the bus. The term “arbitration” refers to a decision to select from among colliding message packets a message packet that should remain on the bus. The selected message packet flows through the bus a head of others, while the other message packet(s) not selected in the arbitration will be transmitted again after a predetermined period of time elapses. More specifically, a message packet with the arbitration field indicating an ID with a higher priority wins in the arbitration and flows on the bus ahead of any other message packet.
Such systems and related techniques are described in, for example, Japanese Patent Laid-Open Publication Nos. 2001-119416 and 2002-176427.
As described above, the ID indicated in the arbitration field of a message packet represents a destination device, and indicates a priority of the message packet flowing on the bus. Thus, in the related art CAN standard, a message packet with a higher priority, which is indicated by the ID of a destination device of the message packet, is allowed to flow through the bus ahead of others. On the other hand, when a collision between message packets occurs on the bus in a system operating under the CAN standard, in some cases, it may be preferable to determine an order of priorities based on IDs of devices from which the message packets are transmitted, and to allow a message packet having an ID of a transmitting device indicating a higher priority to flow through the bus ahead of others. However, under the related art CAN standard, because an order of priorities is determined based on destination IDs indicated in the arbitration fields of message packets, there is a problem in that an order of priorities cannot be determined based on IDs of transmitting devices.